Transplantation of human umbilical cord blood hematopoietic stem cells (HSCs) has become a standard procedure to treat high-risk hematologic malignancies as well as other hematological disorders, and also holds promise as a therapy for certain infectious diseases (e.g. HIV). However, hematopoietic engraftment following cord blood transplantation in clinical settings is frequently suboptimal due to low numbers of transferred CD34+ stem cells, and slow reconstitution of neutrophils is often associated with post-transplant complications. Prior work in murine models and correlative studies of human patients have established that invariant Natural Killer T (iNKT) cells play important roles in promoting both HSC engraftment and myelopoiesis after transplantation, yet the mechanisms by which they mediate these effects remain poorly understood. Preliminary studies have demonstrated that iNKT cells produce cytokines such as GM-CSF, trigger CD40 signaling, and activate purinergic signaling and PGE2 production by other cells - all of which are activities that have been implicated in promoting key hematopoietic events such as HSC self-renewal, migration, and myeloid differentiation. The overarching goal of the studies proposed here is to understand the cellular interactions and molecular signaling pathways engaged by iNKT cells during post-transplant hematopoiesis, and to associate these with particular hematopoietic outcomes. We will use complementary in vitro and in vivo model systems to dissect iNKT- mediated mechanistic processes and to test the effects of human iNKT cells on the engraftment and differentiation of human cord blood HSCs after transplantation into highly immune deficient NSG mice. Since iNKT cells might interact with multiple different cell types and engage multiple different molecular pathways to influence hematopoietic outcomes, Aim 1 will determine the impact of direct iNKT-HSC interactions as compared to the indirect effects of soluble hematopoietic factors produced during iNKT interactions with other cell types, and Aim 2 will determine the impact of specific signaling pathways engaged by iNKT cells that could be utilized in either direct or indirect pathways. These studies will advance the field by distinguishing how iNKT-mediated pathways contribute to different outcomes following HSC transplantation, which will further our ability to design therapeutic strategies that engage these unique innate T lymphocytes. PHS 398/2590 (Rev. 06/09) Page Continuation Format Page